A microlens for an image sensor such as a charge coupled device (CCD) can produce a high-definition image sensor mainly by forming a fine pattern and the fine pattern is produced using a photoresist. Specifically, a microlens is produced by a method including: applying a resist composition containing a polymer resin and a photosensitizer on a substrate to make a film of the resist composition; patterning the resultant film by a photolithography method; and developing the film to form one pattern. Therefore, for the resist composition used as a microlens material, it is required to be highly sensitive and excellent in pattern forming ability. The formed lens pattern is exposed to a high temperature condition in a soldering process, so that for the resist composition, it is also required that the formed lens pattern has a desired curvature radius and has high heat resistance and high transparency.
As one of important characteristics among the above required characteristics, the sensitivity can be mentioned. The enhancement of the sensitivity leads to shortening of the production time in the industrial production and at the present when the demand for the image sensor is substantially increasing in recent years, the sensitivity has become one of extremely important characteristics. When the sensitivity is not satisfactory, a desired pattern cannot be resolved, so that it becomes impossible to form an advantageous lens shape. Although the sensitivity can be enhanced also by enhancing the solubility of a polymer in the material in an alkaline developer, when the composition of the polymer is changed, it influences largely on other characteristics such as a refractive index and hygroscopicity, so that there is a limitation.
As the important characteristics required for the microlens material besides the sensitivity, transparency and heat resistance can be mentioned. Usually, a module such as a camera having a microlens is mounted on a substrate in which an electronic circuit has already been provided. Although for this mounting, a lead-containing solder has conventionally been used, lead is harmful for the human body and the environment, so that in recent years, the use of a lead-free solder containing no lead is advocated. The lead-free solder using a metal other than lead has a melting point higher than that of the lead-containing solder. Therefore, when in the mounting process, a lead-free solder is used, there becomes required a heating treatment at a temperature higher than in the case of using a lead-containing solder. At this time, there is caused the problem that when a microlens is produced with a material having unsatisfactory heat resistance, transparency of the microlens is lowered. Then, in order to obtain a resist composition (microlens material) having high heat resistance, it is required with respect to not only the used resin, but also the used crosslinkable compound, sensitizer, and other additives to be excellent in heat resistance.